The invention relates to the technique and apparatus for pulling single crystals from the melt on the seed by the Czochralski method (hereafter called CZ method), and more particularly relates to the pulling process in which the crucible of the growth furnace is continuously replenished with raw material in order to pull a longer single crystal through a single run of the growth furnace.
The invention may be efficiently used in growing oriented large-sized single crystals of various substances, such as scintillation alkali halide single crystals (e.g. NaI(Tl), CsI(Tl), CsI(Na)) and semiconductor single crystals (e.g. Si, Ge).
According to CZ method a single crystal is grown in the following manner. Starting material is melted in the crucible, and a rotating vertical rod carrying a seed crystal is lowered until the seed comes in contact with the melt. After the lower end of seed crystal is partially melted, the melt temperature is reduced to stop further melting. Then the rod carrying seed crystal is slowly moved up, and the single crystal grows on seed crystal. This method is carried out in a non-continuous manner: that is, the entire amount of raw material required for getting single crystal of particular dimensions is loaded into the crucible at the beginning, and the pulling process is terminated after most of this material has been consumed. The demand for crystals of greater length and greater diameter resulted in a trend toward the use of continuous pulling method (hereafter called CCZ=continuous Czochralski method), in which the crucible is additionally fed with raw material during the growth process.
Together with greater dimensions of the grown crystal, the CCZ method provides a way to get homogeneous distribution of doping impurities along the crystal length, which cannot be achieved by CZ method. The solubility of impurities in the solid material of the growing crystal is significantly less than that in the melt; that is why in the CZ process, where amount of melt in the crucible decreases over time, the concentration of the additive in melt simultaneously increases, so that its concentration in the most recently pulled part of crystal is much higher than that in the part of the crystal pulled first. In continuous CCZ process the amount of the melt in the crucible may be kept constant and the decrease of the concentration of the additive in the melt due to its entering into growing crystal may be compensated by adjusting the concentration of the additive in the material being used to replenish the melt. In this case the constant concentration of the doping impurity in the melt will provide its homogeneous distribution in the whole volume of the grown crystal.
While the CZ and CCZ methods enable the production of single crystals of higher quality compared with those obtained by other known methods of growing crystals, pulling single crystals with preset properties and perfect crystalline structure requires compliance with a number of rigorous conditions such as provision of stability and axial symmetry of the thermal fields in the melt and in the growing crystal, maintenance of preset growth rate and preselected shape of the solid-liquid interface, and assurance of stability of the growing crystal diameter; thus known in the art apparatus are considered below taking into account these requirements.
Known in the art are apparatus comprising a stationary crucible consisting of two communicated containers having a common heater or individual heaters (U.S. Pat. Nos. 3,507,625, 4,547,258 and 5,290,395, for example). Raw material in a particle or powder form is fed to one container, where it is melted, and a rotating single crystal is pulled from the other container. Molten raw material flows through connecting part of the crucible keeping the same melt level in both containers. The disadvantage of these apparatus consist in that an axially symmetric thermal fields in the melt and in the growing crystal cannot be obtained because of lack of axial symmetry of the crucible and impossibility of crucible rotation.
In an apparatus disclosed in U.S. Pat. No. 2,892,739 a crucible consist of two cylindrical containers arranged coaxially with respect to each other with a vertical space between them equal to the difference in their heights and a radial space between them equal to the difference in their radii so that an annular space is defined between two containers to which parades of raw material are fed. The inner container has a hole in its bottom which provides communication between its inner space and the outer annular space. The crucible is mounted on a plate resting on a rotary rod and is surrounded by a cylindrical heater. The crystal pulling mechanism includes a vertical rotary rod with a seed crystal holder on its lower end. A vertical-tube is located over the annular space for feeding with raw material in a particle or powder form. After raw material is melted in the crucible, the growing-process begins in the same manner as described above for CZ method. Concurrently with the beginning of pulling, feeding of raw material is started through the vertical tube to the annular space of the rotating crucible to be uniformly distributed over its periphery. Particles of raw material fall onto the melt surface and are melted, and the melt from the annular space overflows to the inner container to provide continuous feeding of liquid raw material to the melt contained therein.
The similar apparatus is disclosed in U.S. Pat. No. 5,314,667 where a circular partition is centrally positioned in the cylindrical crucible, with a small gap between the lower end of partition and the bottom of crucible, providing overflow of the melt from the annular space (outside partition) to the inner space (inside partition).
The above described apparatus use a coaxial rotation of the crucible and of the crystal holder, and this ensures axial symmetry of thermal fields in the melt and in the growing crystal even though some elements of design, like feeding tube, trend to disturb concentric temperature distribution inside the growth furnace. With the same mass rates of growing and of feeding these apparatus also provide the stable position of the melt surface, what needs to be done first on the way of creation of the preselected shape of the solid-liquid interface.
The main disadvantages of the apparatus described in U.S. Pat. Nos. 2,892,739 and 5,314,667 result from a limited possibility of providing an adequate amount of overheating of the melt in the annular space of the crucible. The melt temperature in this space should be by a certain amount higher than the temperature of the melt in the inner space of the crucible, and the difference in these temperatures should be the greater, the higher the rate of feeding, hence the greater growth rate or growing crystal diameter. Failure to comply with this requirement would result in incomplete melting of raw material fed to annular space. On other hand, the melt temperature in the inner space is fairly close to the melting point and should not be above of certain level which depends on the pulling rate, crystal diameter, vertical temperature gradient near the solid-liquid interface of the growing crystal, etc. Because of a strong influence of the melt temperature in the annular space on the melt temperature in the inner space, this requirement either cannot be complied with at all or it may be met in the above described apparatus with substantial limitations, that is for certain performance parameters only (for example, for lower growth rates or smaller crystal diameters). Moreover, heat radiation loss from the upper end of partition, extending over the melt surface, can result in the occurrence of solidification of the melt on partition's walls. Once solidification starts on the inner wall of partition, solidified material grows toward crystal until touching it, thus interrupting the crystal growing operation. If solidification occurs on the outer wall of partition, it will assist to accumulation in the annular space of solid raw material particles which cannot be melted completely. Another disadvantage of the apparatus described in U.S. Pat. Nos. 2,892,739 and 5,314,667 resides in that outgrows may appear on the walls of annular space because of crystallization on the walls of liquid drops arising from splashes making by raw material particles hitting the melt surface, as well as due to direct incidence of these particles on the walls and their sticking to them. Gradual increase in the outgrows size sooner or later results in complete obturation of the radial gap in the annular space of the crucible, hence in the interruption of replenishing the melt with raw material and interruption of continuous growing process. Additional problem arises when these apparatus are used for growing crystals, which density in a solid form is less than in a liquid form, as in the case of silicon, for example. Thus, solid silicon particles, fallen into annular space, initially float on the molten silicon liquid surface; and, because the heat emissivity of silicon is higher in a solid form than in a liquid form, the solidification of the melt may be developed around these floating particles.
Similar apparatus, where the partition has several holes in its lower portion, providing overflow of the melt from annular space into inner space of the crucible, are described in U.S. Pat. Nos. 5,087,321, 5,087,429, 5,126,114, 5,139,750, 5,143,704 and 5,270,020. To solve the above mentioned problems all these apparatus contain heat insulating covers which are disposed above the molten liquid surface of annular space and above the top of partition. Besides, in some embodiments of these inventions the following additional devices are used:
auxiliary ring heater, positioned above the molten liquid surface outside the partition and under the heat insulation cover (U.S. Pat. Nos. 5,087,321 and 5,126,114);
auxiliary ring heater, positioned above the molten liquid surface of annular space and above the top of partition, without insulation cover above this heater (U.S. Pat. No. 5,087,429);
circular shield facing a meniscus position of the molten liquid at inner walls of the partition to reflect the heat radiation from meniscus portion, if this shield is made of carbon or metal it may be provided with electrodes to permit the supply of electric current and thereby to form it into electric resistance heater (U.S. Pat. No. 5,139,750).
In one of embodiments of U.S. Pat. No. 5,087,429 the partition comprises a resistance heater which is so designed that its outer periphery is heated to a higher temperature than its inner periphery; this immersed in the melt heater provides the greater difference of melt temperatures in the annular space and inside the partition.
Any or all above means aimed at preventing solidification of the melt on partition or crucible walls and at providing the entire melting of raw material particles, falling into the annular space of the crucible, may, of course, enhance the continuous growing process, but they don't eliminate the main cause of all mentioned above disadvantages: strong influence of the melt temperature in the annular space on the melt temperature in the inner space, or, in other words, zones of melting raw material and of growing single crystal are too close to each other. These means also unlikely may be universal, because in case of growing crystals of substances which melt has high vapor pressure (e.g. alkali halides) the condensation of vapors on shields and covers may disturb their normal functioning as well as may cause the accumulation of condensate and its subsequent falling into the melt.
U.S. Pat. No. 5,242,531 comprises the description of apparatus for growing crystals by CCZ method, where zones of melting raw material and of growing single crystal are sufficiently far apart from each other. The housing of the growth furnace contains an additional crucible with its heating elements which is arranged above the annular space of the main crucible from which the single crystal is being pulled. The bottom part of the additional crucible is provided with a capillary through which the liquid raw material can flow approximately in the center of annular space of main crucible. Additional crucible is half-filled with the melt which is replenished by melting in it the solid raw material in a particle or powder form fed from a hopper arranged outside the housing. The feeding rate of melt from additional crucible to main crucible is controlled by the power of heating element surrounding the capillary: if it is high enough the melt can pass through capillary into main crucible, if it is low enough the melt inside the capillary will be frozen and the flow of the liquid will be stopped. This kind of feed maintenance, strictly speaking, is not continuous and, because of alternating solidification and melting raw material in the capillary, can have some restrictions in respect of frequency of discrete feedings, which in some cases can be not high enough to be consistent with really continuous crystal growth. But the main disadvantage of the described apparatus is the lack of axial symmetry of the thermal field around the single crystal being pulled, caused by additional crucible with its heaters. The similar apparatus, having the same disadvantage, is described in U.S. Pat. No. 5,360,480; the housing of the growth furnace in this apparatus also contains additional crucible, of which the melted raw material can flow into a feeding tube over its open upper edge, positioned inside the crucible, when the falling solid raw material causes the raising of the melt level above the edge of that tube.
The other possibility of recharging melted raw material is described in U.S. Pat. No. 4,036,595. Adjacent the housing of the growth furnace there is a second housing of similar structure which has an additional crucible with the melt which is replenished by melting in it raw material fed from a canister disposed above that crucible. A tube, provided with a heating coil, extends from a point intermediate the bottom and the top surface of the melt in the additional crucible through the holes in the side walls of both housings to the top surface of the melt in the annular space of the main crucible from which a single crystal is being pulled. The feeding of the melt through this tube is controlled by building up pressure of inert gas in the second housing and therefore forcing the liquid to flow into the main crucible. When a desired level is reached, the pressure between both housings is equalized and liquid flow ceases. Disadvantages of this kind of recharging melted raw material may result from difficulties to create a reliable seal between walls of housings and incandescent feeding tube (heated above the melting point of raw material), what needs to be done both to provide a reliable control of the feeding rate and to prevent contamination of the melt due to air leaks when the growing is conducted at low pressures.
Another possibility of recharging melted raw material from adjacent additional crucible into the main crucible, that are disposed in two separate housings, is described in U.S. Pat. No. 4,282,184. A heated siphon tube for molten raw material transfer extends from additional crucible through the common side wall of both housings to the main crucible from which a single crystal is being pulled. The opposite ends of this tube are not deeply immersed within the melt contained in the first crucible and within the melt contained in the second crucible. The siphon tube is initially filled with molten raw material by raising pressure of inert gas in the housing surrounding additional crucible. Then, after equalizing the pressures of inert gas in both housings, siphon tube will establish the same level in both crucibles and for continuous growing solid raw material in a particle or powder form is added to and melted in the additional crucible during the process of pulling single crystal from the main crucible. A constant melt level is maintained by an optical monitoring device controlling by the rate of replenishment solid raw material. The apparatus does not use any partition between the growing crystal and the immersed end of siphon tube, and this is its obvious disadvantage, because the hotter molten raw material, supplied by heated to high temperatures siphon tube, may easily reach the perimeter of growing crystal and cause its partial melting. Besides, the lack of partition imposes a limitation on the rotation rate of the main crucible, because at high rotation speed some waves or ripples on the surface of the melt, arising from immersed end of the siphon tube, can reach the growing crystal and degrade its quality.
The closest to present invention known in the art is an apparatus disclosed in U.S. Pat. No. 4,203,951. Schematic representation of this apparatus is shown in FIG. 1. The apparatus comprises a hermetic housing 1 receiving a rotary rod 2 carrying a holder 3 of a seed crystal 4. A crucible 5 consists of an inner container 6 and an outer container 7 communicating through small apertures 8 formed at the level of the bottom wall of the outer container. Crucible is mounted on a support 9 rigidly secured to a rotary rod 10. A lower part of the inner container 6 protruding downwards relative to the outer container 7 is designed to contain melt. A flat bottom heater 11 is disposed under the support 9, and a flat annular side heater 12 is located under the bottom wall of outer container 7. The electric heaters 11 and 12 are mounted on a pedestal 13 made of heat insulating material and are independently controlled. A tube 14 for feeding raw material into the annular space between containers 6 and 7 is received at the top in the housing 1. The vertical axis of the lower portion of the tube 14 is located about in the middle of the annular space of crucible 5, and the lower end face of tube 14 lies slightly below the upper end face of crucible 5. The apparatus also comprises means (not shown) for imparting rotation and reciprocations to the rod 2, for rotation of the rod 10 and for feeding the raw material to a funnel 15 of tube 14. The apparatus according to the invention functions in the following manner. Prior to the operation starting material is charged in the inner container 6 in an amount such after its melting the level of melt 16 should be slightly lower of apertures 8. After raw material is melted and the temperature of the bottom wall of outer container 7 becomes high enough to melt falling on its surface raw material particles, the growing process begins in the same manner as described above for CZ method. Concurrently with the beginning of pulling, feeding with raw material in a particle or powder form is started through tube 14 to the annular space between containers 6 and 7 of the rotating crucible 5 to be uniformly distributed over its periphery. Particles of raw material fall onto the hot bottom surface of outer container 7 and are melted on it, and immediately afterwards the melt overflows through apertures 8 to inner container 6 providing continuous feeding of liquid raw material to the melt contained therein. At the stage of growing of a single crystal 17 from the seed crystal 4 to a preset final diameter, the rate of feeding is gradually increased in accordance with the increase in the mass rate of growth of the single crystal enlarging in diameter. After the preset diameter is achieved, the feeding rate is then maintained at constant value.
Thus, the distinctive feature of this apparatus is a direct continuous flow of raw material to the bulk of the melt, from which the single crystal is being pulled, first in the solid state and then, just after melting, in the liquid state. In the case of growing crystals with doping impurities this feature presents a unique possibility for fixation the shape of the solid-liquid interface in the growing crystal by means of abrupt increasing of the impurity concentration in the bulk of the melt from zero to definite optimal value, with subsequent revealing the interface shape in the grown crystal through the difference in physical properties on each side of former interface. For example, in the case of activated scintillation crystals the first stage of growing process (from the seed to the final crystal diameter) can be performed using pure molten raw material. Then the feeding with pure solid raw material is abruptly changed onto feeding with its mixture with activator, the content of which is large enough to reach a definite optimal concentration in the bulk of the melt in a short period of time. Thereafter and to the end of the growing process the feeding is performed with a mixture having a lesser content of activator which provides its constant concentration in the melt and hence its uniform distribution in the whole rest volume of the grown crystal. Using an UV-lamp one can see in the grown crystal well-defined interface between non-acivated and activated (luminescent) parts of the crystal which represents the exact shape of the former solid-liquid interface in the growing crystal.
The proper shape of the solid-liquid interface can be easily achieved by selection of optimal relationship between the temperatures of heaters 11 and 12. With maintenance of the same mass rates of growth and of feeding at all stages of the growing process, the level of melt 16 in crucible 5 and, hence, the solid-liquid interface do not change their position ensuring a stable thermal field in the vicinity of interface. The axial symmetry of the thermal fields in the melt and in the growing crystal is ensured by rotation of rods 2 and 10.
Nevertheless the apparatus under consideration is not free of disadvantages. The main disadvantage lies in the fact that the radiant heat exchange between the hottest lower part of growing crystal 17 and the cold walls of housing 1 is shielded by the inner walls of outer container 7 rising above the melt level. This makes difficult to remove the heat of fusion and forces to lower the temperature of melt 16 closer to the melting point resulting in limitation of growth rate, especially on the early stage of growing in length when the height of the crystal is less than the distance between the melt level and the upper end face of crucible 5. Another disadvantage comes from the fact that side heater 12 is located too close to the column of melt 16, and because of this there is definite limitation of its maximal allowable temperature resulting in restriction of the rate of melting raw material particles on the bottom of outer container 7, that is in restriction of the feeding rate. Besides, this also imposes restriction on the rate of crucible rotation; really, to avoid accumulation of the solid raw material in outer container 7, any portion of raw material particles fallen from tube 14 onto the bottom of this container must be melted on it for period of time which should not exceed the time of one complete revolution of the crucible, hence the allowable rotation rate should be the lesser the lower the temperature of side heater 12. One more disadvantage of the apparatus described in U.S. Pat. No. 4,203,951 is the lack of partition between the inner walls of crucible and the growing crystal; if the temperature of the melt flowing down from apertures 8 is substantially higher than the temperature of the bulk melt, the growing crystal may be partially melted at its perimeter.